Fuel cells are increasingly being used as a power source in a wide variety of applications. For example, a fuel cell system has been proposed for use in vehicles as a replacement for internal combustion engines, for example. Such a system is disclosed in commonly owned U.S. Pat. App. Pub. No. 2004/0209150, hereby incorporated herein by reference in its entirety. Fuel cells may also be used as stationary electric power plants in buildings and residences, as portable power in video cameras, computers, and the like. Typically, the fuel cells generate electricity used to charge batteries or to provide power for an electric motor.
Fuel cells are electrochemical devices which directly combine a fuel, e.g. hydrogen, and an oxidant, e.g. oxygen, to produce electricity. The oxygen is typically supplied by an air stream. The hydrogen and oxygen combine to form water. Other fuels can be used, e.g. natural gas, methanol, gasoline, and coal-derived synthetic fuels.
One type of fuel cell known in the art is a proton exchange membrane (PEM). The basic components of a PEM fuel cell are two electrodes separated by a polymer membrane electrolyte. Each electrode is coated on one side with a thin catalyst layer. The electrodes, catalyst, and membrane together form a membrane electrode assembly (MEA).
In a typical PEM fuel cell, the MEA is sandwiched between “anode” and “cathode” diffusion mediums (DM's) or diffusion layers that are formed from a resilient, conductive, and gas permeable material such as carbon fabric or paper. The DM's serve as the primary current collectors for the anode and cathode, as well as provide mechanical support for the MEA. The DM's and MEA are pressed between a pair of electronically conductive plates, also known as bipolar plates, which serve as secondary current collectors for collecting the current from the primary current collectors. The plates conduct current between adjacent cells internally of the stack in the case of bipolar plates and conduct current externally of the stack in the case of monopolar plates at the end of the stack.
The bipolar plates each include at least one active region that distributes the gaseous reactants over the major faces of the anode and cathode. These active regions, also known as flow fields, typically include a plurality of lands which engage the DM and define a plurality of grooves or flow channels therebetween. The channels supply the hydrogen and the oxygen to the electrodes on either side of the PEM from an inlet manifold. In particular, the hydrogen flows through the channels to the anode where the catalyst promotes separation into protons and electrons. On the opposite side of the PEM, the oxygen flows through the channels to the cathode where the oxygen attracts the hydrogen protons through the PEM. The electrons are captured as useful energy through an external circuit and are combined with the protons and oxygen to produce water vapor at the cathode side.
The reactants typically include water in the form of water vapor for humidification of the PEM. On the anode side of the bipolar plate, the concentration of water vapor increases at the outlet region due to the consumption of the hydrogen fuel in the cell. Likewise, the concentration of water vapor increases on the cathode side of the bipolar plate due to the formation of water as the hydrogen is oxidized. The water vapor must be managed to inhibit liquid water accumulation within the flow channels.
The water vapor is typically propelled through the flow channels and into an outlet manifold by a velocity of the reactants flowing through the flow channels. The increasing concentration of water vapor at the outlet region facilitates formation of liquid water in the flow channels. Any liquid water that forms in the flow channels is typically propelled through the flow channels and into the outlet manifolds.
A typical flow field includes a quantity of individual flow channels adapted to discharge the water into a common port. The quantity of individual flow channels may combine to form a larger channel in an outlet region. The ports and larger channels generally lead into the outlet manifold and take up an increased cross-sectional area compared to the individual flow channels in the flow field. The increased cross-sectional area taken up by the ports and the larger channels allow liquid water to accumulate. However, the velocity of the reactants flowing through the flow field is often not sufficient to propel the liquid water through the ports and channels and into the outlet manifold. Consequently, the accumulated water can block the flow of the reactants through the channels. The accumulation of water in the channels is typically referred to as “flooding” or “stagnation.” A flooded fuel cell can have reduced electrical power output as the blocked channels starve the fuel cell of the reactants needed to meet a desired electrical output.
It would be desirable to produce a plate for a fuel cell wherein a flow field of the plate is adapted to minimize the accumulation of liquid water at an outlet region of the plate.